\ l J6 \V. JI. WEED — MINERAL VEINS BNBICHBD BY SULPHIDES 



upon the ore deposits of that district.* He has suggested that the sec- 

 ondary deposition or transposition of the copper minerals at the Butte 

 | Montana; mines may have been produced by waters descending from 

 the surface. He says further : 



" Secondary deposition, or transposition of already deposited minerals, lias 

 played an unusually important role. In the ease of the copper veins it has not 

 been confined to the oxidizing action of surface waters, which has resulted in an 

 impoverishment of the ore bodies, but below the zone of oxidation it has resulted 

 in the formation of the richer copper minerals, bornite, chalcocite, and covellite, 

 in part at least, by the breaking up of original chalcopyrite. Unusual enrichment 

 of the middle depths of the lodes has thus been caused. Whether the two processes 

 of impoverishment and enrichment have been differing phases of the action of de- 

 scending waters, or whether the latter may have been a later result of the rhyolite 

 intrusion, has not yet been definitely decided. It is, however, fairly well deter- 

 mined that the enrichment of the copper deposits is so closely associated with the 

 secondary faulting that it may be considered to be a genetic result of it. 



" In the silver veins surface oxidation lias resulted in general in the enrichment 

 of the ore bodies. No certain evidence of secondary enrichment in the sulphide 

 zone of these ore bodies was obtained." 



A very important contribution upon this subject has recently appeared, 

 entitled "A Description of the Pyrite Deposits of the Huelva Region in 

 southern Spain and the adjoining Portions of Portugal," in which Dr 

 J. H. L. Vogtf says that the amount of copper in the pyrite of these ore 

 bodies becomes in a general way less and less with increasing depth at 

 the San Domingo mine. The ore which at the surface has 4 or 5 per 

 cent of copper contains at a depth of 80 meters but 2 per cent. At 100 

 meters the copper contents has fallen to 1? and If per cent, while at 130 

 meters in depth the ore holds only about 1] percent. In the ores of the 

 Dionisio mine the percentage, which is 4 per cent in the upper levels, 

 has fallen to 2 per cent at 200 meters and to 21 per cent at 350 meters. 

 It is to be understood, he says, that the copper contents in different parts 

 of the respective levels will vary somewhat, though the average contents 

 always grow less in depth. 



This impoverishment in depth has also been discussed by Klock- 

 raann.J He coincides with the view held by all the Spanish geologists, 

 and says this constant decrease in depth of the copper contents is due 

 to the fact that the original copper contents of the pyrite of the upper- 

 most part of the vein went into solution through weathering, and that 

 the copper-bearing solution then seeped down along cracks and fissures 

 into the deeper lying pyrites. It there formed a new generation of 



* Butte Special Folio, folio 39, U. S. Geological Survey, lS'JS. 

 t Zeitschrift fur Praktische Geologie, July, 18'JU. 

 X Ibid., 1895, p. 35. 



